In this paper, we consider the H-infinity filtering for networked control system with packet dropout. By using delta operator approach, a new system model is proposed. For a given packet loss rate, the filtering error system is modeled as an asynchronous dynamic system. Sufficient conditions are obtained to illustrate stability of the desired filtering error system in the form of a number of some linear matrix inequalities, method of designing H-infinity filter is also given. A numerical example illustrates that the proposed approach is effective.